Introduction to Earthquake Resistant Design
Design Issues

Design vs. Analysis

The basic analysis paradigm that

Demand < Capacity

depends on having a structure, a realistic analytical model, an accurate computer program, & well defined loading. On the other hand, the design paradigm consists of finding a structure that:

  • Performs well under service and probable environmental actions.
  • Is insensitive to uncertainties in loading.
  • Is economical

Many analysis tools exist, but the structural engineer really needs design tools to come up with a structure that will fulfill the above objectives.

Building Codes as Design Tools

The Building codes are useful design tools - they define the "standard of care" - an important legal and professional concept. However, the focus is on the minimum standards needed for the protection of life safety. The mission of a building code is to

"provide minimum standards to safeguard life or limb, health, property, and public welfare by regulating and controlling the design, construction, quality of materials, use and occupancy, location, and maintenance of buildings."

Difficulties with Using Codes

Professional liabilitiy concerns are increasing - are the simplfied code provisions adequate for complicated projects? In addition, current codes provide little guidance on how to achieve improved performance of new structures. A prescriptive format is used, rather than a performance-based approach. Building owners want better performance, but the code doesn't give engineers any methods for providing it.

The problem is not simply limited to a lack of performance-based criteria. Building codes do not treat many important subjects such as:

  • Much current engineering work focuses on rehabilitation of existing buildings designed using earlier deficient codes
  • Structural engineers are more involved in design of mechanical equipment, bridges, and special structures not covered by the code

In addition, uneven performance of code-compliant buildings has been noted in recent earthquakes. Some buildings performed very well, while the performance of others was inadequate. Examples of inadequate performance in recent earthquakes include:

  • Nearly 70% of new steel buildings shaken by the Northridge earthquake suffered brittle fractures in their welded beam-to-column connections
  • More than 10% of new steel buildings in Kobe collapsed!
  • Several new reinforced concrete structures collapsed or were severely damaged during the Northridge and Loma Prieta earthquakes
  • Many buildings were closed due to damage to architectural elements

As a result, many building owners and public officials are unhappy with performance of code-compliant structures.

Activity to Improve Building Codes

There is much activity to improve codes in order to rectify the inadequacies described above. Improvements are being made on several fronts simultaneously - improvements to existing national codes, as well as the development of new performance based approaches and guidelines for existing buildings. Improvements to national codes include:

  • National Earthquake Hazard Reduction Program (NEHRP) Tentative Provisions for Seismic Design of Buildings (Building Seismic Safety Council).
  • International Building Code (merging of three main model codes used in the U.S. and incorporation of NEHRP provisions.

New approaches which provide an emphasis on improved performance and reliability, as well as rational guidlines for the upgrade of existing buildings include:

  • Performance-based design (EERC/FEMA)
  • Vision 2000 (SEAOC)
  • SAC Steel Project (FEMA)
  • Next generation codes (NSF, FEMA)
  • Guidelines for the Rehabilitation of Existing Buildings (FEMA 273)

The Current Situation

The current situation is in flux! The new codes are in development, but aren't ready for use yet. In fact, they will probably take more than a decade to stabilize and become consistent. And the revised national codes may not fully define the state of practice in a particular area. Local groups are likely to write special provisions to accommodate local conditions. In addition, the developments do not all have the same performance objectives:

  • If the objective is life safety: how safe?
  • Performance states may differ
  • What is meant by performance state (e.g. continued occupancy) may differ between codes
  • Target confidence levels vary